1. Field of the Invention
This invention relates generally to the use of a zero-valent metal emulsion to remediate halogenated solvents found in water. Preferably, zero-valent iron emulsions containing nanoscale or microscale iron particles are used to remediate dense non-aqueous phase liquid (DNAPL) sources found in groundwater.
2. Description of the Background Art
Remediation of halogenated solvents, such as trichloroethylene (TCE), halogenated hydrocarbons, and other chlorinated solvents, is of great concern due to their toxicity and their persistence in the environment. Halogenated solvents, such as TCE, enter the groundwater and soil environments through improper disposal practices. These halogenated solvents are used in industry as degreasers; in the production of dry cleaning fluids, spot removers, insecticides and pesticides, as well as in many other manufacturing processes. Because of halogenated solvents"" wide variety of uses they have become ubiquitous in the environment. According to the U.S. Environmental Protection Agency (USEPA), TCE has been found in at least 852 of the 1430 National Priorities List sites (ATSDR 1995).
When released into the ground, halogented solvents, such as TCE, will sink through the subsurface soil and groundwater until it is contained by a non-permeable surface such as bedrock. At this point it will pool and slowly dissolve into any water that it comes into contact with. Halogenated solvents, such as TCE, that have higher densities than water are referred to as dense nonaqueous phase liquids (DNAPLs). Due to the low solubility of many halogenated solvents, for example, TCE""s low solubility (1.1xc3x97103 mg/L), the pool will continue to contaminate groundwater for extended periods of time. As the groundwater is in constant motion, this pool can contaminate very large areas of potential drinking water. Breakdown of the halogenated solvents in natural environments is very slow and produces other potential harmful by-products that are also regulated by the USEPA in Title 40 Code of Federal Regulations. Currently, the maximum contaminant level of TCE acceptable in ground water established by the EPA is 4-5 xcexcg/L.
Traditionally, the method of choice for remediation of TCE has been accomplished by pumping the contaminated water to a surface plant and removing the TCE by air stripping or granular activated carbon adsorption. The decontaminated water is then disposed of into wastewater treatment plants or re-injected into the ground.
Pump-and-treat technology has many limitations. Installing the surface plant is very costly. Additionally, although the initial depletion of TCE is quite high, the depletion levels off to values that are sometimes above the regulatory levels. The surface plant requires constant monitoring; it produces hazardous wastes, and requires an energy source to operate the pumps and strippers. Due to TCE""s low solubility in water, remediation of the groundwater using pump-and-treat technologies will take very long periods of time (e.g. decades) in order to maintain protection of human health and the environment. The pump and treat technologies primarily provide containment, rather than remediation. Because of the length of time necessary in the pump and treat technologies, high operation and maintenance cost over the time period of remediation are incurred.
Several pilot and full-scale projects for remediation of DNAPLs employ the use of a permeable reactive barrier wall (PRBW) placed within groundwater. The PRBW is installed across a path of a contaminated plume. The contaminants are removed or degraded producing decontaminated water on the down gradient side of the wall. The use of zero-valent metals, such as iron, to reductively dechlorinate DNAPLs has been employed as the reactive material in these PRBWs. The use of PRBWs has several advantages over the traditional pump-and-treat methods of remediation. This process produces little waste and is much less labor intensive. Since it is a passive system, mechanical failures are eliminated. The most prominent drawback of the use of an in-situ permeable reactive wall is that, like pump-and-treat systems, it never actually treats the contaminant pool. These processes rely on the DNAPL dissolution and transport for treatment. Again, the process of complete remediation will take an extended period of time.
Currently, there are no available proven technologies that can treat 100% of DNAPL sources. These sources include free-phase, residual phase, and sorbed (or matrix diffused) phases of DNAPL. Attempts have been made to remove the DNAPL sources through heating to enhance volatilization. Such heating techniques have included steam injection and radio-frequency-heating. However, this approach is limited because of the energy costs associated with heating the groundwater and the exponential volume of areas that will need to be treated to ensure that the entire DNAPL source is encountered and treated.
An alternative approach has been to flood the source area with surfactants or oxidizing agents. DNAPL contaminates are remediated by injecting a surfactant to either solubilize or mobilize the DNAPL pool. The presence of surfactant micelles increases the solubility of the DNAPL in the groundwater. This method of remediation is unique in that it actually confronts the pools of DNAPL. However, DNAPLs such as TCE are more subject to uncontrolled migration using this technique and could produce larger contamination zones. Additionally, these surfactants only travel through most permeable zones. DNAPL pools diffuse into geological areas of low permeability preventing their 100% removal that is required to prevent the remaining DNAPL from re-contaminating the groundwater.
Therefore, a critical need exists for technologies that can effectively treat DNAPL sources in the saturated zone and result in both their destruction and containment with reduced treatment times and lower costs.
To overcome the foregoing problems, the present invention comprises a zero-valent metal emulsion containing zero-valent metal particles, surfactant, oil and water, and a method of using the same, to enhance dehalogenation of dense non-aqueous phase liquid (DNAPL) sources. The zero-valent metal emulsion is particularly suited for dehalogenation of solvents including, but not limited to, trichloroethene (TCE) and other halogenated hydrocarbons.
In a preferred embodiment, microscale and nanoscale iron particles are used as the zero-valent metal particles. Microscale and nanoscale iron particles are excellent reactive media to incorporate into a preferred zero-valent iron emulsion due to their reactivity, low cost, and natural presence in the subsurface. However, other zero-valent metal particles and combinations may be used to dehalogenate a DNAPL source. For example, iron particles doped with palladium are useful zero-valent metal particles to dehalogenate DNAPLs. Also, a variety of bimetallic particle combinations are useful in dehalogenating DNAPL sources.
Food grade vegetable oils and various cationic, anionic and nonionic surfactants are preferred components in the generation of the zero-valent metal emulsion. Preferably, food-grade surfactants are used because of their low toxicity.
In the preferred zero-valent iron emulsion, a very active zero-valent iron emulsion contains 32-53 wt. % oil, 36-59 wt % water, 6.4-10.6 wt. % iron particles, 1.0-1.8 wt. % surfactant More preferably, the zero-valent iron emulsion contains 42.7 wt. % oil, 47.4 wt. % water, 8.5 wt % iron particles, 1.4 wt. % surfactant. However, other ranges of oil, water, iron particles, and surfactant may also be effective to dehalogenate DNAPLs.
The zero-valent metal emulsion that is generated is hydrophobic, which allows the DNAPL source, for example TCE, to enter through an oil membrane where it can diffuse to the zero-valent metal particle and undergo degradation. In contrast, an aqueous slurry of reactive iron particles would be rejected by the hydrophobic DNAPL pool.
The zero-valent metal emulsion efficiently degrades DNAPLs, such as TCE, and challenges the DNAPL pool. The preferred zero-valent iron emulsion containing zero-valent nanoscale or microscale iron particles reductively dehalogenates DNAPLs to non-toxic hydrocarbons, such as ethene. The effectiveness of the degradation may be determined by comparing the rate constants of degradation of DNAPLs, such as TCE, of pure zero-valent metal particles to the rate constants of the zero-valent metal emulsion.
The zero-valent metal emulsion may be delivered to the DNAPL phase in a variety of ways. Ideally, the DNAPL phase would be located and defined. In one embodiment, the zero-valent metal emulsion is delivered in-situ to contamination pools via a system of injection wells. The injection wells are permanent structures that are left in the ground for repeatedly injecting the zero-valent metal emulsion into the ground. Alternatively, the zero-valent metal emulsion may be delivered to the DNAPL using direct push technology. This technology includes push rods that are advanceD specified distances into the injection site. The zero-valent emulsion is delivered to the DNAPL through holes in a distal portion of the push rods. When the injection of the zero-valent metal emulsion is complete, the push rods are removed from the ground. It is also possible to deliver the zero-valent metal emulsion by way of slurry injection into a soil matrix.
The present invention overcomes the previous understanding that the incorporation of zero-valent metal particles, such as iron particles, into a liquid membrane micelle would lead to passivation of the particle surface with regard to its ability to dehalogenate compounds. Kinetic studies have shown that the dehalogenation rates of a zero-valent metal emulsions are very high, and in fact, are much higher than free zero-valent metal particles with regard to the dehalogenation pools of pure DNAPL.
A beneficial feature of the zero-valent metal emulsion is that no halogen-containing atoms exit from the micelle during remediation. The zero-valent metal emulsion draws free DNAPL into the inside of the micelle where the degradation reaction takes place. For example, during the remediation of TCE, no chlorinated daughter-products have been found to exit from the micelle. The only degradation by-products that have been detected are hydrocarbons, such as ethene, which are easily degraded by biological action and are non-toxic.
Additionally, the zero-valent metal emulsion is simple to prepare and is relatively inexpensive. The zero-valent emulsion is made from environmentally compatible components. The preferred surfactant is of the food-grade quality, and the liquid membrane preferably consists of a vegetable oil which is biodegradable. Since the zero-valent metal emulsion can be injected into the DNAPL zone by using simple push wells and incur no continuing operating costs, use of an zero-valent emulsion possesses an economic advantage over a long-term pump and treat methodology. Because of the thousands of DNAPL sites in the United States alone, use of this technique would generate millions of dollars in economic improvement within the remediation community.
As discussed in the previous section, the present invention comprises a zero-valent metal emulsion containing zero-valent metal particles, surfactant, oil and water, and a method of using the same, to enhance the dehalogentation of dense non-aqueous phase liquid (DNAPL) sources. Although the present invention is particularly suitable for the dechlorination of trichoroethene (TCE), other DNAPL sources may likewise be remediated using the subject zero-valent metal emulsion.
The zero-valent metal emulsion contains a surfactant stabilized biodegradable oil-in-water emulsion with zero-valent metal particles contained within emulsion micelles. In one preferred embodiment, a zero-valent iron emulsion containing zero-valent nanoscale iron particles or microscale iron particles is used to dehalogenated DNAPLs. However, other zero-valent metal particles and combinations may be used, including various bimetallic particle combinations and, more specifically, iron particles doped with palladium. In the preferred zero-valent iron emulsion, a very active zero-valent iron emulsion contains 32-53 wt. % oil, 36-59 wt % water, 6.4-10.6 wt. % iron particles, 1.0-1.8 wt. % surfactant. More preferably, the zero-valent iron emulsion contains 42.7 wt. % oil, 47.4 wt. % water, 8.5 wt % iron particles, 1.4 wt. % surfactant. However, other ranges of oil, water, iron particles, and surfactant may also be effective to dehalogenate DNAPLs as shown in the Examples below.
Zero-valent metal particles have been proven to effectively degrade halogenated solvents. For example, the mechanism and reaction rates of which iron reduces chlorinated aliphatics has been studied extensively due to iron""s low cost and low toxicity. The half reaction of (Fe0) to (Fe+2) as seen in Equation 1 has a reduction potential of xe2x88x920.440V. The estimated standard reduction potentials of alkyl halides at a pH of 7, as in Equation 2, ranges from +0.5 to +1.5V. Therefore, the net reaction (Equation 3) is thermodynamically favorable.
xe2x80x83Fe0xe2x86x92Fe2++2exe2x88x92xe2x80x83xe2x80x83(1)
RX+2exe2x88x92+H+xe2x86x92RH+Xxe2x88x92xe2x80x83xe2x80x83(2)
Fe0+RX+H++Fe2++RH+Xxe2x88x92xe2x80x83xe2x80x83(3)
These equations indicate that the iron assisted reductive dehalogenation of the chlorinated solvents is a corrosive process.
Additionally, the pathways of the dehalogenation of DNAPL""s such as TCE have been proposed. TCE undergoes hydrogenolysis where the replacement of each of the three chlorines occurs sequentially. TCE reduces to cis-1,2-dichloroethene, trans-1,2-dichloroethene, and 1,1-dichloroethene. These intermediates in turn reduce to vinyl chloride, ethene and ethane.
In use, DNAPL sources diffuse through the oil membrane of the zero-valent metal emulsion whereupon they reach the surface of the zero-valent metal particles where dehalogenation takes place. A hydrocarbon reaction by-product of the dehalogenation reaction, for example ethene, diffuses out of the emulsion micelle and vents to the aquifer.
The zero-valent metal emulsion may be delivered to the DNAPL phase in a variety of ways. Ideally, the DNAPL phase would be located and defined. In one embodiment, the zero-valent metal emulsion is delivered in-situ to contamination pools via a system of injection wells. The injection wells are permanent structures that are left in the ground for repeatedly injecting the zero-valent metal emulsion into the ground. The injection wells may contain screen portions through which the zero-valent metal emulsion may pass in order to contact the DNAPL phase. Alternatively, the zero-valent metal emulsion may be delivered to the DNAPL using direct push technology. This technology includes push rods that are forced into the injection site. A distal portion of the push rods has a series of holes along its length for delivering the zero-valent emulsion. The push rods are advanced further into the soil depending on the amount and depth of the contamination. When the injection of the zero-valent metal emulsion is complete, the push rods are removed from the ground. It is also possible to deliver the zero-valent metal emulsion by way of slurry injection into a soil matrix. This process decreases the need for long-term treatment and monitoring of the contaminated areas.